What is the ruling on adding words of love and pet names between spouses in religious narratives to illustrate the fruit of piety in the marital relationship, and are such narratives considered forbidden romanticism?

There is no objection to what you have asked about, nor does it contradict Sharia, especially given the soundness of the intention and its conformity with Sharia meanings, and the rarity of these terms and states, and their susceptibility to conveying other meanings and intentions.